1. Field of the Invention
The present invention relates to an end face structure of an optical fiber that emits laser light, to an optical fiber laser including such an end face structure, and to a laser processing apparatus including such an optical fiber laser.
Priority is claimed from Japanese Patent Application No. 2004-120006, filed Apr. 15, 2004, which is incorporated herein by reference.
2. Description of Related Art
In recent years, lasers have been employed in various fields, including material processing apparatuses, medical treatment appliances, and measuring instruments. Especially in the field of material processing apparatus, laser light has been widely used because high-precision machining is made possible with laser light having a very small beam spot and a high power density since laser light exhibits excellent focusing property, because non-contact processing with little damage to a workpiece is possible, and because processing of hard materials that absorb laser light is possible. Specifically, laser light is used for welding, cutting, laser marking, high-precision processing, or the like.
CO2 lasers using carbon dioxide gas as a lasing medium, solid-state lasers using a Nd:YAG crystal as a lasing medium, lasers that are obtained by converting wavelength of the laser light of solid-state lasers using a nonlinear optical crystal, or the like, have been conventionally used as lasers for processing according to particular applications.
On the other hand, optical fiber lasers that employ, as a lasing medium, optical fibers in which silica glass, which is doped with at least one rare earth element (or ions), such as Er, Nd, Yb, Ho, Tm, or the like, is used as a host glass (hereinafter, referred to as “rare-earth doped optical fibers”), or optical fibers made of fluoride glass, are attracting attention. An optical fiber laser has the following advantages: it has high laser light generating efficiency; it is possible to reduce the size of a laser apparatus since an optical fiber that is used as a lasing medium has a large surface area and can be cooled by air; and the same material can be used for both a lasing medium and a laser light propagation medium. Accordingly, optical fiber laser is being used for material processing apparatuses medical treatment appliances, or the like.
In such fields, although a laser having high output power in which an average output on the order of kilowatts is used in some applications, optical fiber lasers having relatively low average output power of 100 W or less are used in the fields of laser marking, or the like.
FIG. 1 is a schematic diagram of a typical pulse fiber laser apparatus. This optical fiber laser generally includes a pumping part and a cavity part. The pumping part includes a pumping light source 1 and a condenser lens 3. Pumping light 2 output from the pumping light source 1 is focused on an incident end face 5 by the condenser lens 3, and is incident on a rare-earth doped optical fiber 6 that is a lasing medium. The cavity part includes a rare-earth doped optical fiber 6, a cavity mirror 4, an output coupler 11, a collimating lens 8, and a Q-switch 10. A dielectric multilayer film that selectively reflects laser light without reflecting the pumping light 2 is provided on the cavity mirror 4. Another dielectric multilayer film that reflects some of the laser light and transmits the other laser light is provided on the output coupler 11. The rare-earth doped optical fiber 6 is adjusted the length thereof so that the desired output characteristics are obtained, and the two ends of the rare-earth doped optical fiber 6 are optically polished. The pumping light 2 that is incident on the dearth doped optical fiber 6 is absorbed by the ions of the rare earth element or elements that have been doped into a core of the rare-earth doped optical fiber 6, and the ions of the rare earth element or elements that absorb the pumping light 2 emit light having a particular wavelength. The light emitted from the ions of the rare earth element or elements propagates through the rare-earth doped optical fiber 6 while being amplified, and is emitted from an emitting end face 7. The emission light 9 from the emitting end face 7 is collimated by the collimating lens 8, and is shaped into a desired pulse shape by the Q-switch 10. Some of the emission light 9 is reflected by the output coupler 11. The reflected emission light 9 is fed back to the rare-earth doped optical fiber 6, reflected by the cavity mirror 4, and emitted from the emitting end face 7. In this cycle, the reflected emission light 9 goes back and forth within the cavity. In one round-trip path, if amplification of the output light is larger than loss of the output light 9, laser light 12 is output from the output coupler 11. In addition, if the Q-switch 10 is not employed, such a light is output as a continuous laser.
Since the diameter of the core of the rare-earth doped optical fiber is generally in the range of several to tens of micrometers, the power density of the light propagating through the optical fiber core is significantly high. In a case in which the output power is 1 kW and the diameter of the core is 10 μm, for example, the power density of the light output from the core can be as high as about 1.3 GW/cm2. With such a high power density, burn-in caused by adhered dust or contaminants may destroy the end face of the optical fiber, which may result in critical damage. Thus, the emitting end face 7 may be damaged in the fiber laser shown in FIG. 1, which is one of the leading factors inhibiting realization of a high output optical fiber laser.
One technique for preventing such a damage to end faces is known. In this technique, the power density is reduced, i.e., the spot diameter is enlarged at the emitting end face 7 (for example, Japanese Unexamined Patent Application, First Publication No. 2002-40271).
FIG. 2 is a cross-sectional diagram illustrating such an end nice structure in detail. In this figure, an optical fiber 13 is secured in a ferrule 15 with an adhesive, and the end face of the optical fiber, including a core 14, is optically polished. The ferrule 15 in which the optical fiber 13 is secured is halfway inserted in a capillary 16, and the remaining hollowed portion of the capillary 16 is filled with an optical filler 17, for example, a ultraviolet light (UV) curing resin. In this structure, the light emitted from the optical fiber 13 propagates through the optical filler 17 with the spot diameter thereof being enlarged, and when the light reaches the end face of the optical filler 17, the spot diameter is increased so as to be larger than the diameter of the core. For example if the spot diameter at the end face of the optical filler 17 is twice as large as the diameter of the core, power density at the end face of the optical filler 17 is reduced to a quarter of the power density at the end face of optical fiber 13. Accordingly, burn-in can be prevented even when a high output power laser is employed, regardless of the type of laser, i.e., pulsed light, or continuous light.
In addition, in the case of a pulsed laser, a high peak power of the output pulse is desired, and to increase the peak power, it is essential that an end face of a rare-earth doped optical fiber be subjected to anti-reflection treatment of laser light. In a laser apparatus as shown in FIG. 1, for example, when pumping light is incident on the rare-earth doped optical fiber, rare earth ions doped into the core of the rarer-earth doped optical fiber are pumped, thereby inducing population inversion. If the emitting end face of the rare-earth doped optical fiber is not provided with the anti-reflection treatment, spontaneously emitted light emitted from the rare earth ions is reflected by the emitting end face, and such reflected light is amplified while propagating through the core. In some cases, laser oscillation may occur, which results in a decrease in the population inversion. In contrast, even in a case when an anti-reflection treatment is provided on the emitting end face, generation and amplification of spontaneously emitted light occurs, which, however, does not result in laser oscillation. Accordingly, high population inversion can be maintained. When the loss of the Q-switch is abruptly reduced, the light reflected from the output coupler propagates in the rare-earth doped optical fiber and laser oscillation drastically occurs. Thus, pulses having high peak power can be obtained. An anti-reflection treatment is typically applied by providing on the end face of the optical fiber a dielectric multilayer film that does not reflect laser light.
However, in a conventional end face structure of an optical fiber shown in FIG. 2, an optical filler that has the same refractive index as the optical fiber should be used in order to prevent reflection at the end face of the optical fiber. Accordingly, expensive filler are used since there are not many options for filler materials, which results in increased production cost.
In addition, when an ultraviolet curing resin is used, if the length L of the optical filler 17 shown in FIG. 2 is long, the curing resin is not completely cured since ultraviolet light does not reach the region far from the surface and air bubbles are present. It is difficult to form an optically complete continuous junction. Furthermore, the resin exhibits poor resistance to optical power, and the resin may be burned
Furthermore, providing a dielectric multilayer film in the anti-reflection treatment requires vacuum processing, which is time-consuming and expensive.